1. Biofilters provide waste treatment by removing uneaten fish feed.

2. Biofilters convert ammonia present in fish waste to nitrate.

3. Biofilters increase phosphorus as a nutrient for fish in water. 

How many of the statements given above are correct?

(a) Only one

(b) Only two

(c) All three

(d) None.

8. Ans: b

Explanation:

About Recirculatory Aquaculture System (RAS )

• Instead of the traditional method of growing fish outdoors in open ponds and raceways, in RAS system fish are typically reared in indoor/outdoor tanks in a controlled environment. 
• In RAS water is recycled and reused by passing it through mechanical and biological filters and after suspended matter is removed. 
• The steps in RAS include solids removal, ammonia removal, CO2 removal and oxygenation. 

Biolfilters: A biological filter consists of various material such as plastic beads (bioball), net fiber, molluscan shells, fiberglass, ceramic or rock that has large amounts of surface area for colonizing the nitrifying bacteria cells which grow on all surfaces of the biofilter media.

How does filtration works?

To remove these wastes, the mechanical filters and biofilter or biological filters are used. 

Mechanical filtration: Mechanical filtration of the outlet water from the fish tanks is used for the removal of suspended organic and inorganic wastes. The removal of solid waste including uneaten fish feed is the first stage in serial filtration design. Smaller particles of organic sediments are consumed by microscopic organisms including bacteria. But not all the organic matter is removed in the mechanical filter; the finest particles will pass through the filter along with dissolved compounds such as phosphate and nitrogen which are then removed using biofilters.

Hence, statement 1 is correct.

Biolfilteration: The breakdown of organic matter and ammonia is a biological process carried out by bacteria in the bio-filter. Heterotrophic bacteria oxidize/breakdown the left organic matter by consuming oxygen and producing carbon dioxide and ammonia. Nitrifying bacteria convert ammonia into nitrite and finally into nitrate. The bacteria that convert ammonia to nitrite are known collectively by their genus name Nitrosomonas. Like ammonia, the nitrite produced by the Nitrosomonas bacteria is toxic to aquatic organisms and must be oxidized further to a less toxic form of nitrogen. This is accomplished by genus of bacteria called Nitrobacter.

Hence, statement 2 is correct. 

Efficiency of bio-filtration depends primarily on Water temperature, oxygen and pH level. To reach an acceptable nitrification rate, water temperatures should be kept within 10 to 35 °C (optimum around 30 °C) and pH levels between 7 and 8. A lower pH level reduces the efficiency of the bio-filter. Under anaerobic conditions, hydrogen sulfide (H2S) can be produced, especially in saltwater systems. This gas is extremely toxic to fish, even in low concentrations, and fish will be killed if hydrogen sulfide is generated in the system. 

Note: Phosphate is an inert substance, with no toxic effect, but nitrogen in the form of free ammonia (NH3) is toxic and needs to be transformed into non-toxic form in the bio-filter.

How Phosphates are removed from RAS?

In recirculating aquaculture systems, cultivated fish cannot incorporate all the phosphate contained in the feed. Excess phosphate ends up in the culture water and in the sludge produced, which can harm other aquatic life in the area by causing algae blooms. One method of phosphorus removal is adsorption (adsorbent RhizoSorb (base material of aluminum oxide) is used) which is a surface phenomenon that is often used to bind dissolved pollutants to a solid-phase medium and remove them from water. Thus biofliters reduce the amount of phosphorus in water.

Hence, statement 3 is incorrect.

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